Periodically-focused traveling-wave tube



May 23, 1961 D. J. BATES ET AL PERIODICALLY-FOCUSED TRAVELING-WAVE TUBE 4 Sheets-Sheet 1 Filed Oct. 2. 1958 D. J. BATES, H.R. JOHNSON, 0. T. PURL.

AGFNT y 23, 1961 D. .1. BATES ET AL 2,985,792

PERIODICALLY-FOCUSED TRAVELING-WAVE TUBE Filed Oct. 2, 1958 4 Sheets-Sheet 2 7, flu k INVENTORS,

o. J. BATES, H. R. JOHNSON, 0.1. PURL.

y 1961 D. J. BATES ET AL 2,985,792

PERIODICALLY-FOCUSED TRAVELING-WAVE TUBE! Filed Oct. 2. 1958 4 Sheets-Sheet 3 INVENTORS, D J BATES, H.R.JOHNSON. 0. T. PURL WfW 5 gm 0 m AGENT May 23, 1961 D. J. BATES ET AL PERIODICALLY-FOCUSED TRAVELING-WAVE TUBE 4 SheetsSheet 4 Filed Oct. 2, 1958 INVENTORS, 0.1 was. we JOHNSON. o T PURL .E 8. 0 2 555 E 3. #2 0m. .v on. my Vitae 0: mm.

United States Patent 2,985,792 PERIODICALLY-FOCUSE? TRAVELING-WAVE TUB David J. Bates, Los Angeles, Horace R. Johnson, Paio Alto, and Oliver T. Purl, Menlo Park, Calif., assignors to Hughes Aircraft Company, Culver City, Calif., a corporation of Delaware Filed Oct. 2, 1958, Ser. No. 764,884

6 Claims. (Cl. 3*153.5)

This invention relates generally to microwave devices and particularly to a high power periodically focused traveling-wave tube amplifier.

In traveling-wave tubes an electron stream interacting with a propagating electromagnetic wave is utilized to increase the level, or amplify, the electromagnetic energy. In order to achieve effective interaction between the stream of charged particles and the electromagnetic wave energy, the phase velocity of the electromagnetic wave is decreased by means of any one of a number of different types of slow-wave structures. The classical example of such structures is a helix wound about the path of the electron stream. Microwaves traversing the length of the helix do so at substantially the speed of light. However, their axial velocity is decreased by substantially the ratio of the axial helix length to the circumferential length of the wound conductor. 7

Another type of slow-wave structure particularly useful at higher power and higher frequencies is the folded waveguide or interconnected cell type of slow-wave structure. In this type of structure a waveguide is effectively wound back and forth across the path of the electron stream. This provides, as with the helix, a path of propagation which is considerably longer than the axial length of the structure and hence the traveling wave may be made effectively to propagate at nearly the velocity of the electron stream. The interactions of the electrons of this stream and the traveling wave causes velocity modulations and bunching of the beam. The net result may then be a transfer of electromagnetic energy from the electron beam to the wave travelling on the slow-wave structure.

The present invention is primarily but not necessarily concerned with traveling-wave tubes utilizing slow-wave structures of the type last above mentioned, viz., the folded waveguide or interconnected cell type. Modern practical techniques for fabricating this type of slow-wave structure usually provide a series of interaction cells or cavities disposed adjacent each other sequentially along the axis of the tube. The electron stream passes through each cell along the axis of the tube and electromagnetic coupling is provided between the cavity and the electron stream. Each cavity is also coupled to an adjacent cavity by means of a coupling hole in the end wall defining the cavity. Generally, in the past, these coupling holes between adjacent cells have been alternately disposed on opposite sides of the axis. When coupling holes are so arranged, the folded waveguide type of energy propagation may be visualized as involving energy traversing the length of the tube which enters each cavity from one side, crosses the stream and leaves the cavity from the other side, thus traveling a sinuous or tortuous extended path.

In practical traveling-wave tubes the electron stream is projected along the axis of the tube through minimum sized holes in the interaction cells or cavities or, more generally, as proximate as possible to the slow-wave structure for maximum interaction therewith. Accordingly, the stream must be focused and constrained to such apath by focusing means to prevent excessive impingement of electrons A 2,985,792 Patented May 23, 1961 k on the slow-wave circuit. This is generally done by immersing the electron stream in a strong axial magnetic field which tends to provide the required constraint so that the electron stream may pass as closely as possible to the slow-wave structure without excessive interception of the electrons by the slow-wave structure. Generally, in the past, such an axial constraining magnetic field has been provided by aligning the traveling-wave tube concen trically within a long solenoid wound of a conductor carry ing relatively large electrical currents. As traveling-wave tubes have been designed and developed for higher and higher power handling capabilities, such solenoids have had to be larger and larger and carry more current,- thus" requiring not only very large auxiliary power supplies but also special cooling means. Further, if the focusing solenoid in such arrangements fails, even momentarily, to sup,- ply the focusing magnetic field, the electron stream may immediately destroy the slow-wave structure or other parts of the traveling-wave tube due to the large current in the electron stream. Obviously, the weight and bulk of a complete traveling-wave tube, including its solenoid focus{ ing system, make such a tube impractical for many mobile operations. 7 Another common scheme for focusing a traveling-wave tube involves the use of large permanent magnets with a' pole piece at each end of the traveling w'ave tube to provide a strong axial magnetic field. Here too, the size and weight of the magnet are prohibitive for many airborne or other mobile applications. 7 There has relatively recently been developed a system for periodically focusing traveling-wave tubes. In accord? ance with this system, generally, there is provided external to the electron stream and to the slow-wave structure, and to the vacuum envelope therefor, a series of axially short permanent magnets disposed about the periphery of the envelope sequentially along the length of the tube. Placed between adjacent ones of these magnets is a pole piece which extends radially inwardly as nearly as possible to the envelope. The axial gap between adjacent pole pieces is used to provide a magnetic lens, the focusing effects of which extend into the slow-wave structure for constraining the electron stream. Again, as higher and higher power is desired in the traveling-wave tube such a scheme becomes less effective in providing adequate focusing because insufiicient magnetic material can be placed between adjacent pole pieces or because the pole pieces themselves saturate and provide inadequate lens fields. A formidable wall against further development has been reached when the pole pieces have been placed to provide gaps asclosely aspossible to the slow-wave structure and the maximum amount of magnetic material has been provided between the pole pieces. In other words the best conventional magnetic system which produces the field for the lenses cannot be brought close enough to the electron stream, thus limiting the magnetic field produced on theaxis to a value inadequate for focusing high power electron streams. It is accordingly an object of the present invention to provide a traveling-wave tube capable of very high power operation without danger of destruction of the tube due to an inadequately focusedel'ectron stream.

It is another object of the present invention to provide a high power traveling-wave tube which is exceedingly light in weight, rugged and dependable in its physicalconstruction, and which does not require auxiliary power supply means for supplying current to a focusing solenoid. It is another object to provide a high power,"high gain traveling wave tube which is not susceptible todelete'rious oscillations;

It is another object to provide a traveling wavetulie having arugged metallic and ceramic envelope".

It is another object to provide a traveling-wave tube which is periodically focused and which makes more eflicient use of the periodic magnets.

It is another object to make integral the magnetic pole pieces of the periodic magnets with the end walls of the radio frequency interaction cavities along the tube.

It is another object to provide efficient nonrefiective coupling between adjacent interaction cells along the length of the tube.

It is another object to provide eflicient nonreflective coupling between the interconnected cell slow-wave structure of the tube and an external transmission line.

It is another object to provide an interconnected cavity type slow-wave structure which provides a more efiicient interchange of energy between the electron stream and the traveling waves propagating along the slow-wave structure.

It is another object to provide a unitary traveling-wave tube which functions to provide high overall gain while at the same time not suffering disadvantages of instability or oscillations.

Briefly, these and other objects are achieved in accord ance with the present invention in the following ways.

An interconnected cell, folded waveguide or other filter type of periodic slow-wave structure, for example, may use periodic focusing magnets and interposed ferromagnetic pole pieces. The ferromagnetic pole pieces for the periodic focusing magnets may be constructed to extend radially inwardly to the periphery of the electron Stream to form the end walls of the interaction cavities of the slow-Wave structure. The periodic focusing magnets may be annular disc-shaped elements, which are disposed between the adjacent pole pieces. Substantially immediately within each magnet is a short hollow conductive nonmagnetic spacer cylinder, the inner surface of which defines the radial extent of each interaction cell, with the ferromagnetic pole piece extensions forming the interaction cell end walls. The pole pieces and the spacer cylinders may be self-jigging or aligning, and stacked and hermetically bonded together during assembly to form a rugged all-metal interaction structure and envelope for the traveling-wave tube. The inner portions of the iron pole pieces may be surfaced with a highly conductive material, for example, copper or silver which have a resistivity of the order of 10' ohm-centimeters, while that of iron is of the order of 10" ohm-centimeters, thus providing a continuing slow-wave structure. Coupling holes are provided in each piece to provide coupling between adjacent interaction cells. Nonrefiective efiicient coupler sections are provided for coupling the slow-wave structure to an external transmission line. The microwave energy enters a particular one of the interaction cells and may then effectively diametrically traverse the cell to a coupling hole on the opposite side from whence the energy is propagated further along the slow-wave structure. Among the features of this arrangement are the establishment of an extremely effective magnetic focusing field, due to the close proximity of the pole pieces to the electron stream, and the provision of an easily fabricated and simply combined structure. Furthermore, this arrangement permits the variation of the slow-wave structure so as to achieve significant operating advantages.

In accordance with another feature of the invention the traveling-wave tube may be severed into a plurality of amplifying sections, each one of which is substantially isolated from the others except by electron stream coupling. Thus, although the composite tube may provide extremely high gain, no one amplifying section will have sufiicient gain to be unstable with regard to deleterious oscillations.

Further novel features of this invention, as well as the invention itself, both as to its organization and method of operation, will best be understood from the following description, taken in conjunction with the accompanying drawings which are not drawn to scale and in which like reference numerals refer to like parts, and in which:

Fig. 1 is an overall view partly in longitudinal section and partly broken away of a traveling-wave tube constructed in accordance with the present invention;

Fig. 2 is a detailed longitudinal sectional view of a portion of the tube illustrated in Fig. 1;

Fig. 3 is an exploded view of a set of typical elements included in the structure of an embodiment of the present invention; and

Fig. 4 is a detailed exploded view of a typical isolator section of the traveling-wave tube of Fig. 1.

Referring to the drawings and their description, a number of features are shown for purposes of completeness of discussion of a traveling-wave tube according to the present invention, which features are not claimed in the present application but are claimed and described more fully in applications assigned to the assignee of the present application and filed concurrently herewith: Self Aligning Traveling-Wave Tube and Method by T. Leonard and T. J. Flannery, Serial Number 764,886, now Patent No. 2,957,102 granted October 18, 1960; Severed Traveling- Wave Tube by D. J. Bates and O. T. Purl, Serial Number 764,883, which discusses in greater detail and claims the structure illustrated in part in the present Figs. 2 and 3; and Periodically Focussed Traveling-Wave Tube With Tapered Phase Velocity by D. J. Bates, Serial Number 764,885, now Patent No. 2,956,200, granted October 11, 1960.

Referring with more particularity to Fig. 1, there is shown a traveling-wave tube 12 utilizing a plurality of annular disc-shaped focusing magnets 14. In the example of this figure, these are permanent magnets and are diametrically split, as shown in later figures, to permit their being easily slipped between assembled adjacent ones of a series of ferromagnetic pole pieces 16, which are also shown in more detail in the later figures. The system of pole pieces 16 and magnets 14 form both a slow-wave structure and envelope 18.

Coupled to the right hand or input end of the slowwave structure 18 is an input waveguide transducer 20 which includes an impedance step transformer 22. A flange 24 is provided for coupling the assembled traveling wave tube 12 to an external waveguide or other microwave transmission line (not shown). The construction of the flange 24 includes a microwave window (not shown) transparent to radio frequency energy but capable of maintaining a pressure differential for maintaining a vacuum within the traveling-wave tube 12. At the output end of the tube 12, shown in the drawing as the left-hand end, an output transducer 26 is provided which is substantially similar to the input impedance transducer 20.

An electron gun 28 is disposed at the right hand end, as shown in the drawing, of the traveling-wave tube 12 and comprises a cathode 30 which is heated by a filament 32. The cathode 30 has a small central opening 34 to aid in the axial alignment of the gun assembly with the remainder of the traveling-wave tube 12. The cathode 30 is secured about its periphery by a cylindrical shielding member 36 which is constructed in a manner to fold cylindrically, symmetrically back upon itself to form a double cylindrical shield and an extended thermal path from the cathode 30 to its outer supporting means. Such support and an electrical, highly conductive path to the cathode is thus achieved while providing considerable thermal insulation for the cathode and filament due to the extended or tortuous path for heat conduction, as well as because of the multiple cylindrical shielding against radiant heat which is provided by the cylinders shown. For additional details of this type of gun construction, see the patent to J. A. Dallons, No. 2,817,039, entitled Cathode Support, issued December 17, 1957, and assigned to the assignee of the present invention.

A focusing electrode 38 supports the cylindrical shielding member 36. The focusing electrode 38 is generally maintained at the same potential as that of the cathode .to a hollow ceramic supporting tube 48. tube 48 further thermally insulates the inner intensively 30 and is shaped to focus the electron stream emitted .by the cathode in a well-.collimated, high perveance beam of electrons which traverses the slow-wave structure 18 and electromagnetically interacts with microwave energy being propagated therealong. The electron gun configuration is in accordance generally with the teachings in the Patent No. 2,817,033, by G. R. Brewer, which issued December 17, 1957, entitled Electron Gun, which is assigned to the assignee of the present invention, and to which reference may be made for a more detailed explanation. The focusing electrode 38 is in turn supported by a hollow cylindrical support 49 which extends from the periphery of the focusing electrode to the right hand end of the traveling-wave tube 12'. Its opening is hermetically sealed with a metal to ceramic seal 42 by means of a sealing flange 14 made of a material having a low coefficient of thermal expansion, such as Kovar. The right hand extremity of the cylindrical support 40 is supported by an annular flange member 46, which also may be of Kovar, andwhich is sealed in turn The ceramic heated members of the electron gun 28 and also provides electrical insulation between the cathode-beam focusing assembly and the higher potential accelerating anode 52. Substantially encasing the electron gun 28 and secured to the central or radio frequency structure of the traveling-wave tube 12 is a hollow cylinder 50,

which-may be Kovar, to which is sealed the ceramic cylinder 48, thus completing the vacuum envelope about the right hand end of the traveling-wave tube 12.

collector electrode is supported within the end of a water fiacket cylinder 64 which is in turn supported by an end plate 66. A water chamber 68 is thus formed between the outer surface of the collector electrode 62 and the inner cylindrical surface of water jacket 64. A water input tube 70 supplies cool water to this chamber and a water output tube 72 exhausts the heated water out of the water chamber 68. Thus, considerable power may be dissipated without destruction of the collector electrode. Although water has been specified, obviously, other liquids or gases may be-used .as coolants.

The end plate 66 is sealed to a supporting cylinder 74, which may be of Kovar, and which is in turn sealed to a ceramic insulating cylinder 76. This ceramic insulating cylinder 76 is sealed at its opposite end to another Kovar supporting cylinder 78, which is in turn supported and sealed to the slow-Wave structure end disc 80. The collector 62, the end plate 66, the supporting cylinders 74 and 78 and the ceramic insulating cylinder 76 are all coaxially supported in alignment with the axis of the traveling-wave tube 12.

For vacuum pumping or out-gassing the traveling-wave tube 12, a double-ended pumping tube 86 is connected to both of the input and output waveguide transducers 20 and 26. Out-gassing during bake-out of the entire traveling-wave tube 12 is thus achieved as rapidly as possible. After the out-gassing procedure, the tube 86 is separated from the vacuum pumping system by pinching off the tube at the tip $8.

The traveling-wave tube of the present invention may be severed into a number of amplifying sections 90, 92,

-94, '96 and 98. Each of the amplifying segments or sections is isolated from the others by an isolator or terstantially ,cornplete radio frequency isolation between adjacent sections of the slow-wave structure 18'while at the same time allowing the electron-stream to pass straight through the entire length of the traveling-wave tube 1,2, Each amplifying section thus provides an ,optimum gain while providing freedom from oscillations due to ,re; generation. The loss in gain due to each of these isolation sections is of the order of a few decibels and ,is a 'low price to pay for the large overall gain and power handling capabilities of a traveling-wave tube constructed in accordance with the present invention. It should be noted that although the isolation sections provide substantially complete radio frequency isolation between adjacent amplifyingsections, the electron stream is modulated at the output of each amplifying section. The stream thus modulated, as it enters the subsequent amplifying section, launches a new wave therein which is further amplified by the interaction between the new traveling wave and the electron stream. Thus there is provided unidirectional coupling through the electron stream between adjacent amplifying sections.

Referring with more particularity to Fig. 2, there is shown a detailed sectionalview of a portion of the travel, ing-wave tube of Fig. .1. The ferromagnetic pole pieces 16 are shown to extend radially inwardly to approximately the perimeter of the axial electron stream. Disposed contiguously about the electron stream in each case is a short drift tube 110. The drift tube is in the form of-a cylindrical extension or lip protrudingaxially along the stream from the surface of the pole piece 16.

Adjacent ones of the drift tubes 110 are separated by ;a gap 111 which functions as a magnetic gap to provide a focusing lens for the electron stream and also as ,an

electromagnetic interaction gap to provide interaction between the electron stream and microwave energy traversing the slow-wave structure.

At a radial distance outwardly from the drift tubes 119 each of the pole pieces 16 has a second short cylindrical extension 114 protruding from its surface. The extension 114 provides an annular shoulder concentric aboutthe axis of the tube for aligning the assembly of the component elements of the slow-wave structure 18. Disposed radially within the extension 114 is a conductive, nonmagnetic circuit spacer 116 which has the form of an annular ring having an outer diametersubstantially equal to the inner diameter of the cylindrical extension 114.

The axial length of the spacer 116 determines the axial length of the microwave cavities .118 which are interconnected along the length of the slow-wave structure 18.

-It is thus seen that the slow-wave structure maybe ascedure, -a sealing material, such as a brazingalloy, is

placed. When the slow-wave structure 18 is assembled,

,it may-be placed in an oven within a protectivev non-oxidizingatmosphereand heated so that the brazing alloy in the channels 120* melts and fuses or brazes the adjacent membersof the slow-wave structure 18 together to for-tn a vacuum tight envelope. The spacers 1.16 are fabricated of a nonmagnetic material, such as copper, thus woulding a highly conductive cavity wall, while not magnetically shorting out the focusing gaps 112. The entire-interior surfaces of the cavities are preferably plated with a highly conductive material such as a thin silveror gold "plating 121.

For interconnecting adjacent interaction cells,-,a coupling hole 122 is provided in each of the ferromagnetic pole pieces 16, the more detailed shape and orientatiqn of which will be described in connection with the description of Fig.3 below. Also disposed betweenqadjacent-pole pieces 16 are the focusing magnets 14 which are annular in shape and fit angularly or azimuthally symmetrically about the cylindrical shoulder extensions 114. The magnets 14' may be diametrically split to facilitate their being applied to the slow-wave structure 18 after it has been otherwise assembled. The axial length of the magnets adjacent pole pieces 16, and their radial extent is approximately equal to or may be, as shown, greater than that of the pole pieces 16. To provide the focusing lenses in the gaps 112, adjacent ones of the magnets 14 are stacked with opposite polarity, thus causing a reversal ofbthe magnetic field at each successive lens along the tu e.

Referring to a typical isolator section 100, there is shown a substantial continuity of the pole piece-magnetspacer assembly. However, the pole pieces 124 at either end of the isolator section and the spacer 126 are somewhat modified, with respect to pole piece 16 and spacer 116 respectively, which will be shown with greater clarity in Fig. 4. It is sufficient here to point out that attenuating material, which may be in the form of lossy ceramic buttons 128 which extend from within a coupling hole 122 through the special spacer 126 and partially into the wall of the pole piece 124 opposite the coupling hole. The spacer 126 forms a pair of modified cavities 130 which lie opposite respective ones of the coupling holes 122 and which are substantially filled with the lossy attenuating material.

The two cavities 130 are substantially isolated from each other by a short circuiting vane, shown in a later figure, and are isolated from interaction with the electron stream by means of a central portion of the special spacer which has the form of a ring having substantially the same radial dimensions as the drift tubes 110 and which extends between two of the drift tubes 110 as shown, in a manner to substantially shield the electron stream from the slow-wave structure in the region of the isolator section 100.

Along the length of the slow-wave structure 18, individual ones of the pole pieces 16 are spaced by axial distances as represented by a, b, c, and d. In a preferred arrangement of the traveling-wave tube of the present invention, these distances and the associated length of the spacers 116 may be slightly varied with respect to each other so that the effective axial length of the interaction cavities is successively increased toward the output or collector end. This is done in order to decrease the axial phase velocity of the traveling waves so that the desired interaction between the electron stream and the traveling waves will continue to a maximum degree even though the electrons are slowed down toward the collector end.

Referring to Fig. 3, one set of the plurality of pole pieces, magnets and spacers is shown for purposes of describing more clearly how the individual elements of the slow-wave structure 18 are fabricated and assembled. A

typical pole piece 16 is shown twice in the figure, once in plan and once in side elevation. A typical magnet 14 and a typical spacer 116 are shown in side elevation only.

Referring to the side elevation view of the pole piece 16, the orientation of the pole piece 16 concentrically about the electron stream is shown. Substantially immediately surrounding the electron stream is the short drift tube 110 which extends axially in both directions normal to the plane of the pole piece 16. The remainder of the pole piece extends radially outwardly from the drift tube 110 as shown. Positioned radially in between these two extremes are the cylindrical shoulder extensions 114 which extend axially outwardly from both faces of the pole piece 16.

The outer diameter of the cylindrical extension 114 supports the focusing magnet 14 coaxially about the electron stream, while the inner diameter of the extension 114 rests against the outer periphery of the spacer 116. The inner diameter of the spacer 116 determines the outer dimension of the interaction cell which is formed between adjacent ones of the pole pieces 16. Before assembly, a sealing material is placed in the channels 120, which are continuous annular grooves in the end surfaces of the spacers 116.

An off-center coupling hole 122 is provided through tit) 8 each of the pole pieces 16 to provide the transfer of radio frequency energy from cell to cell along the slow- Wave structure 18.

The size, shape and orientation of the coupling hole 122 may be more clearly seen in the plan view thereof at the left hand end of Fig. 3. The drift tube 110 is shown as having an inner radius r slightly larger than the radius of the electron stream and having an outer radius r which substantially defines the inner radius of the interaction cell. The kidney-shaped coupling hole 122 may be formed by an end mill having a diameter extending from r to 13,. The end mill is pressed through the thickness of the pole piece 16 centered upon the arc of a circle 132. The end mill, or preferably the work, may then be swung along this are keeping its center on the circle 132. The work is rotated through an arc of angle a where a may be any angle between zero degrees and, for example, approximately 60. Thus, the kidneyshaped coupling hole 122 lies between a radius r and r, and has circuler ends of diameter ta -r Disposed radially outwardly from the coupling hole 122 is a cylindrical shoulder extension 114, the inner radius of which is designated r and is substantially equal to the outer radius of the spacer 116. The inner radius r of the spacer 116 determines the outer dimension of the radio frequency interaction cell. The outer radius of the extension 114, designated as r is substantially equal to the inner radius of the magnet 14. The outer radius of the pole piece 16 is designated r and the outer radius of the magnet 14 is designated r For angular alignment purposes during assembly, one or more sets of holes 134 are provided through the pole pieces 16 to hold them in a predetermined angular position with respect to each other. A reference notch 136 may be provided on the periphery of each of the pole pieces 16 in order that one may always know from an observation of the outer surface of the assembled tube what the angular orientation of each pole piece is. In the example described here, the notch is always provided opposite the center of the kidney-shaped coupling hole 122.

Referring to Fig. 4, there is shown an exploded view of a typical one of the isolator sections shown in dotted lines in Fig. l, for example, the isolator section 109. The isolator pole pieces 124 are shown in perspective to point out the manner in which they are modified from the typical circuit pole pieces 16. A pair of overlapping circular recessions 136 are provided in the face of each of the isolator pole pieces 124 toward the middle of the isolator section 100. The circular recessions 136 extend approximately half-way through the pole piece 124 and retain the enlarged head portions 138 of the attenuator buttons 128. The attenuator buttons 128 may be formed of a porous ceramic impregnated with carbon. This may be done by soaking the ceramic in a carbohydrate solution, such as sugar, and then baking the soaked piece in an oxygen-free atmosphere to leave a residue of carbon distributed uniformly throughout the volume of the ceramic.

The focusing magnet 14 is typical of the remainder of the focusing magnets and need not be specially modified for the isolator section. The special isolator spacer 126 fits radially within the cylindrical shoulder extensions 114 and has a pair of cavities 130 one each associated with a coupling hole 122. A web end portion 140 closes the end of each of the cavities 130 except for a pair of overlapped openings 142 which are oriented respectively concentric with the circular recessions 136, but have a lesser diameter. The attenuator buttons 128 extend then from the depth of the recessions 136 through the openings 142 in the web end portion 140 through a cavity 130 to approximately half-way through the opposite coupling hole 122.

A circular shoulder 146 is provided on each side of the spacer 126 to receive the end of the drift tube 110 from each of the pole pieces. It is thus seen that the two tcavities 130 are isolated from :each other 'by a :conductive mid-portion .or vane 150. The microwave energy in the slow-wave structure '18 to the left :in the drawing of the isolator spacer 126 may enter coupling hole 122 of the left hand isolator pole .piece shown :in Fig. 4 and will intercept the ends of .tWo of the attenuator :buttons 128 approximately half-way through the coupling .hole 122. Whatever fraction of the microwave energy is not absorbed and dissipated in that portion of the lossy ceramic may pass on into the associated cavity 130 where it will .eventually be completely absorbed.

In exactly the .same manner, microwave :energy in the slow-wave structure to the right of the isolator section and traveling toward the isolator section will be substantially completely absorbed by the other termination.

In the operation of the traveling-Wave :tube 12, microwave energy traverses from right ;to left along the slowwave structure, being amplified first in section 98 due to its interaction with the electron stream. Near the output Of this amplifying section, the traveling wave has grown and has caused considerable density modulation in the electron stream. At the first isolator section, section 106 in the drawing, the radio frequency energy .inthe slow-Wave structure 18 is substantially completely absorbed. However, the modulated electron stream passes on into the next amplifier section, section .96, where it launches a new traveling'wavein that section. The new traveling wave grows and is amplified by the electron stream :until reaching ;its output end at the isolator section 104. The electron stream is further modulated and the :RF energy in .the :slow wave'estructure is again completely absorbed. This procedure is repeated until the highly modulated electron stream :enters the output amplifier .section 90 through the isolator section zltl-and launches a high energy traveling wave upon the output section 90 .of the .slow-wave'structure 18. The output of this "final section .is fed into the output waveguide through the transducer 26.

The isolator sections 100, 102, 104 and :106 each represent a'loss of a few decibels of amplification. However, overall they vastly increase the amountzof power amplification or .gain which may .beachieved Vina single traveling-wave tube. The isolation sections isolate adjacent amplifying sections, thereby to preclude instability and oscillations due to reflections and to too great an amplification in a single traveling-wave tube section.

There has thus been describedanovel traveling-wave tube which combinesin one typical'element thereof a radio frequency slow-wave structure interaction cell and aperiodic focusing magnetic lensstructure. .Some of the many advantages incumbent in the structure to be claimed below are set forth above in the introduction. Many others will become apparent tothoseskilled in'the .art who take advantage of the technological advances described here and incorporated in traveling-wave tubes constructed in accordance with these teachings. Other and additional inventive features may 'be incorporated into traveling-wave tubes of this character, for example: impedance matching devices for broadening the band .of operation and improving the coupling efficiency between the slow-wave structure and an external transmission line; means providing greater-stability with regard to undesired oscillations such as those caused from excessive interaction between the electron beam and higher order perturbed cavity modes or waveguide-modesassociated with a periodic or nearly periodic filter type circuit; means for tapering the slow-wave circuit in order to maximize interaction between the decelerating electron stream and the traveling waves; and means for providing-automatic selfalignment in the assembly in this type of slow-wavestructure.

We claim:

1. A traveling-wave tube structure comprising --means for projecting a stream of electrons along apredetermined path; means for providing a series of electromag? netically intercoupled interaction :cells arranged along and "in electromagnetic interaction relationship with said electron stream and .for providing a series of magnetic lenses along and contiguously about said stream for' focusing and constraining it 'to flow along :said path, said interaction cells having :at each :end thereof a paramagnetic disc element extending radially from said electron stream to apredetermined outer diameter, and a spacer element hermetically sealed between adjacent ones of said disc elements and being relieved to form the interaction volume about said electron stream, and being disposed axially alongand symmetrically thereabout, the outerextremity of said spacer element being substantially less distant from said stream than said predetermined diameter 10f saididisc; a plurality of permanent magnets, each disposed between a different adjacent pair of said disc elements and radially between the associated annular spacer and approximately said predetermined outer radius; and coupling means associated with individual? oneso'f said disc elements and being disposed radially between saidelectron stream and the inner diameter of said spacer element for coupling electromagnetic travel-- ing waves between adjacent ones of said interaction cells..

2. A traveling-wave tube structure comprising means for projecting a stream of electrons along a predeter'-- mined path; means providing a'series of electromagneti cally intercoupled interaction cells arranged along and? in electromagnetic interaction relationship with said elec tronistream and also-providing a series of magnetic lensesalong and contiguously about said stream for focusing and constraining it to flow along said path, said means including at each end of said interaction cells a para-- magnetic disc element extending radially from said :elec-- tron stream to a predetermined outer diameter, said. means also including spacer elements individually, her metically sealed between adjacent ones of said disc ele-- ments and being relieved to form the interaction volume: about said electron stream and being disposed axially,. symmetrically thereabout, the outer extremity of said spacer elements being substantially less distant from said. stream than said predetermined diameter of said discs,. anda plurality of permanent magnets, each disposed be-- tweendifferent wadjacent ones-of said disc elements and. radially between said annular'spacer and approximately" said predetermined outer :radius, the axial spacing be-- tween adjacent ones of said disc elements being varied? along the length of said tube to provide a longer axiali distance between adjacent ones of said disc elements toward the output end of said traveling-Wave tube.

3. A severed traveling-wave tube comprising: means for projecting a stream of electrons along a predetermined path; means providing a plurality of groups of a series :of electromagnetically intercoupled interaction cells arranged along sand in electromagnetic interaction relationship'with saidselectron'stream and also providing a series 'of magnetic cooperating lenses along and contiguously about saidrstream .for focusing and constraining it .to flow along said path-individual ones of said interaction :cells ihaving at each-end thereof a paramagnetic disc elementextending radially-from said electron stream to a-pre'determined outer -diameterya plurality of spacer elements,ea'chi'hermetically sealed between adjacent ones of said idisczelements 'and being radially'inwardly relieved to .form the inter-action volume about said electron stream, :and being disposed axially. symmetrically thereabout, the outer extremity of each spacer element being substantially less distant 'from 'said stream than said predetermined diameter-of said discs; anda-pluralityof permanent magnets for focusingsaidstream disposed -between adjacent ones 'of-said disc elements symmetrically radially coutwardlytfromindividual ones of saidspacer element; .and means :for electromagnetically isolating in- .dividual ones of:said:plura1ity of:groups from each other;

n4. 5A: traveling-wave :tube comprising: an electron gun structure positioned at a first end thereof; means for projecting an electron stream of predetermined diameter along the longitudinal axis of said traveling-wave tube; input means including an input microwave transmission means for providing to the first end of said travelingwave tube a microwave signal to be amplified; output means including an output microwave transmission means for extracting amplified energy from said traveling-wave tube; a plurality of magnetic disc members coaxially positioned along the electron stream path and being substantially equally spaced therealong and each comprising an inner axially extended drift tube portion defining a central aperture for passage of the electron stream, each of said disc members including an inner web portion extending radially outwardly from said drift tube and having a coupling aperture therethrough, adjacent ones of said coupling holes being angularly staggered to preclude a straight through mode of propagation for electromagnetic traveling waves; especially conductive surfacing disposed over the outer surface of said drift tube and over the inner web portion of said disc members for providing a continuing surface of high electrical conductivity along the surfaces defining the slow-wave structure of said traveling-wave tube; a plurality of conductive, nonmagnetic annular cylindrical spacers, each concentric with said longitudinal axis and positioned between adjacent ones of said disc members at a radial position such that the inner surface of said spacers defines the outer cylindrical surface of the radio frequency interaction cells between adjacent ones of said disc members, said spacers being hermetically bonded along their ends to said disc members and maintaining the axial positioning of said disc members along said tube, and a plurality of split, annular permanent magnets concentric with the longitudinal axis of said tube and at least coextensive with the outer portions of said disc members, each of said split, annular magnets having an axial length along said tube substantially equal to that of the spacer between respective successive ones of said disc members and substantially registering with the outer extremity of the inner web portion of the adjacent disc members, each of said split, annular magnets being positioned between a different successive pair of said disc members and being magnetized substantially parallel to said longitudinal axis to form magnetic poles of opposite polarity on the axial extremities of said drift tubes for providing a focusing field for the electron stream passing contiguously therewithin.

5. A traveling-wave tube comprising: an electron gun positioned at one end thereof; means for projecting an electron stream of predetermined average diameter along the longitudinal axis of said traveling-wave tube; input means including an input microwave transmission means for impressing upon the input end of said tube a microwave signal to be amplified; output means including an output microwave transmission means for extracting an amplified signal from said traveling-wave tube; a plurality of electrically conductive magnetic disc members axially positioned along the electron stream path and being spaced apart by an axial distance which is effectively varied to provide a tapered slow-wave structure whereby as the electron stream is decelerated it continues to interact to a maximum degree with the microwave signal traversing the length of said traveling-wave tube, each of said discs comprising an inner axially extended drift tube portion defining a central aperture for passage of the electron stream, adjacent ones of said drift tubes being axially separated to permit interaction between said microwave signals and said electron stream, each of said disc members also including an inner web portion extending radially outwardly from said drift tube and having a coupling aperture therethrough, adjacent ones of said disc members being angularly displaced by rotation with respect to each other, thus providing an angular, staggered relationship between adjacent ones of said cou- 12 pling holes to preclude a straight through mode of propagation with said microwave signal; a plurality of conductive nonmagnetic annular cylindrical spacers, each concentric with said longitudinal axis and hermetically sealed between adjacent ones of said disc members at a radial position such that the inner surfaces of said spacer define the outer cylindrical surfaces of the radio frequency interaction cells between adjacent ones of said disc members; highly electrically conductive surfacing disposed over the outer surfaces of said drift tubes and over the inner web portion of said disc members for providing a continuing surface of high electrical conductivity along the surfaces defining the slow-wave structure of said traveling-wave tube; and a plurality of split, annular permanent magnets concentric with the longitudinal axis of said tube and at least coextensive with the outer portions of said disc members radially beyond the outer surfaces of said spacers, said split annular magnets having an axial length along said tube substantially equal to that of the spacer between respective ones of said disc members and substantially registering with the outer extremity, each of said split annular magnets being positioned between a different successive pair of said disc members and being magnetized substantially parallel to said longitudinal axis to form magnetic poles of opposite polarity on the axial extremities of adjacent respective ones of said drift tubes, thus providing periodic focusing fields for the electron stream passing contiguously within said drift tubes.

6. A traveling-wave tube comprising: an electron gun positioned at one end thereof; means for projecting an electron stream of predetermined average diameter along the longitudinal axis of said traveling-wave tube; input means including an input microwave transmission means for impressing upon the input end of said tube a microwave signal to be amplified; output means including an output microwave transmission means for extracting an amplified signal from said traveling-wave tube; a. plurality of electrically conductive magnetic disc members axially positioned along the electron stream path and being spaced apart by an axial distance which is effectively varied to provide a tapered slow-wave structure whereby as the electron stream is decelerated it continues to interact to a maximum degree with the microwave signal traversing the length of said traveling-wave tube, each of said discs comprising an inner axially extended drift tube portion defining a central aperture for passage of the electron stream, adjacent ones of said drift tubes being axially separated to permit interaction between said microwave signals and said electron stream, each of said disc members also including an inner web portion extending radially outwardly from said drift tube and having a coupling aperture therethrough, adjacent ones of said disc members being angularly displaced by rotation with respect to each other, thus providing an angular, staggered relationship between adjacent ones of said coupling holes to preclude a straight through mode of propagation with said microwave signal; a plurality of conductive nonmagnetic annular cylindrical spacers, each concentric with said longitudinal axis and hermetically sealed between adjacent ones of said disc members at a radial position such that the inner surfaces of said spacer define the outer cylindrical surfaces of the radio frequency interaction cells between adjacent ones of said disc members; highly electrically conductive surfacing disposed over the outer surfaces of said drift tubes and over the inner web portion of said disc members for providing a continuing surface of high electrical conductivity along the surfaces defining the slow-wave structure of said traveling-wave tube; a plurality of split annular permanent magnets concentric with the longitudinal axis of said tube and at least coextensive with the outer portions of said disc members radially beyond the outer surfaces of said spacers, said split annular magnets having an axial length along said tube substantially equal to that of the spacer between respective ones of said disc members and substantially registering with the outer extremity, each of said split annular magnets being positioned between a diiferent successive pair of said disc members and being magnetized substantially parallel to said longitudinal axis to form magnetic poles of opposite polarity on the axial extremities of adjacent respective ones of said drift tubes, thus providing periodic focusing lenses for the electron stream passing contiguously within said drift tubes; and termination means disposed between adjacent groups of said interaction cells for electromagnetically isolating said groups from each other ex cept by electron stream coupling.

References Cited in the file of this patent UNITED STATES PATENTS Schlesinger Jan. 30, 1940 Pierce Apr. 28, 1953 Woodyard Sept. 22, 1953 Wang Apr. 10, 1956. Chodorow Nov. 19, 1957 Yasuda July 15, 1958 Pierce Aug. 12, 1958 

